Removal of dust in urea finishing

ABSTRACT

Disclosed is a method for the removal of urea dust from the off-gas of a finishing section of a urea production plant. the method comprises subjecting the off-gas to quenching with water so as to produce quenched off-gas. The quenched off-gas is subjected to humidification by mixing said quenched gas stream with a humidification fluid selected from (a) saturated steam and (b) superheated steam mixed with a second aqueous stream, so as to produce a humidified gas stream, subjecting said humidified gas stream to particle separation (i.e., dust removal) by means of a scrubbing liquid in which at least part of the particles in the gas stream are soluble.

FIELD OF THE INVENTION

The invention is in the field of urea production, and pertains to theremoval of urea dust from the off-gas associated with the production ofsolid urea particles (urea finishing). Particularly, the inventionpertains to the reduction of the emission of urea dust occurring fromsuch a urea plant finishing section. The invention also pertains to aurea production plant, and to revamping an existing urea productionplant.

BACKGROUND OF THE INVENTION

Urea is produced from ammonia and carbon dioxide. Today's ureaproduction involves relatively clean processes, particularly low in theemission of urea dust and ammonia. However, besides the chemicalsynthesis of urea, the production of urea on a commercial scale requiresthat the urea be presented in a suitable solid, particulate form. Tothis end, urea production involves a finishing step in which a urea meltis brought into the desired particulate form, generally involving anyone of prilling, granulation, and pelletizing.

Prilling used to be the most common method, in which the urea melt isdistributed, as droplets, in a prilling tower and whereby the dropletssolidify as they fall down. However, the end-product is often desired tohave a larger diameter and higher crushing strength than the oneresulting from the prilling technique. These drawbacks led to thedevelopment of the fluidized bed granulation technique, where the ureamelt is sprayed on granules that grow in size as the process continues.Prior to the injection in the granulator, formaldehyde is added toprevent caking and to increase the strength of the end-product.

In order to remove the energy released during crystallization, largeamounts of cooling air are fed to the finishing unit. The air thatleaves the finishing section contains, inter alia, urea dust. With aview to increased demand for urea production, and increasing legal andenvironmental requirements as to reduce the level of emissions, it isdesired that the urea dust is removed, and according to ever increasingstandards.

Over the past several decades the control of air pollution has become apriority concern of society. Many countries have developed highlyelaborate regulatory programs aimed at requiring factories, and othermajor sources of air pollution, to install the Best Available ControlTechnology (BACT) for removing contaminants from gaseous effluentstreams released into the atmosphere. The standards for air pollutioncontrol are becoming increasingly stringent, so that there is a constantdemand for ever more effective pollution control technologies. Inaddition, the operating costs of running pollution control equipment canbe substantial, and so there is also a constant demand for moreefficient technologies.

The removal of urea dust is challenging per se, since the amounts ofoff-gas (mainly air) are enormous, whilst the concentration of urea dustis low. A typical airstream is of the order of 750000 Nm³/h. A typicalconcentration of urea dust therein is about 0.5-1 wt. %. Further, partof the urea dust is of a submicron size. Satisfying current standardsimplies the need to remove a major part of this submicron dust.

A further problem is that the large amounts of air needed in ureafinishing, results in this part of the production process being arelatively costly effort due to the need for very large extractor fanshaving a high electricity consumption. Particularly, when the air issubjected to scrubbing in order to reduce the emission of urea dust, andspecifically a major part of the submicron dust, into the atmosphere, arelatively large amount of energy is simply lost in the process, as aresult of the inevitable pressure drop in the scrubbing device.

In the art it is acknowledged that aerosol-particles (in the sub-micronand micron size range as is typical for urea dust) grow due tocondensation of water on them from supersaturated gas that surroundssuch particles. If the gas surrounding the aerosols/particles issaturated or sub-saturated, but not supersaturated, there is no growthor even negative growth of water from the wet surface of theaerosol-particle. As a result, the particle remains the same size oreven evaporation from the surface of the particle occurs. The generalbelief is that the degree of supersaturation (known as a factor S) needsto be larger than unity (1) to obtain condensation of water on aerosols,which is imperative to obtain growth of particles. Throughout the art onremoving sub-micron dust particles, it is acknowledged that effectiveremoval requires an atmosphere in which water vapor is present in asupersaturated state.

Specifically in the art of urea finishing, such as in urea-granulationtechnology, it is recognized that it is impossible, in practice, toobtain a supersaturated gas-stream downstream of the finishing step.This can be explained with reference to the large amount of relativelydry air, and thus low presence of amounts of water, that are naturallypresent in the off-gas from urea finishing (e.g. from the granulator).In fact, the system initially (in the finishing section) starts fromalmost zero saturation, i.e., too strongly an undersaturated situationto reach a level of saturation, let alone supersaturation.

Condensation scrubbing is a relatively recent development in wetscrubber technology. Most conventional scrubbers rely on the mechanismsof impaction and diffusion to achieve contact between the particulatematter (PM) and liquid droplets. In a condensation scrubber, the PM actsas condensation nuclei for the formation of droplets. Generally,condensation scrubbing depends on first establishing saturationconditions in the gas stream. Once saturation is achieved, steam isinjected into the gas stream. The steam creates a condition ofsuper-saturation and leads to condensation of water on the fine PM inthe gas stream. The large condensed droplets are then removed by one ofseveral conventional devices, such as a high efficiency mist eliminator.[Air Pollution Control Technology Fact Sheet EPA-452/F-03-010].

A background reference on the application, in industrial dustcollection, of particle enlargement by condensation, is Yoshida et al.,Journal of Chemical Engineering of Japan, 1978, Vol.11, No.6, p.469-475. Herein humidification of a gas stream is brought about by steaminjection. This is taught to be especially effective for low temperaturegas.

Another such background reference is EP 555 474. Herein a process isdisclosed for cleaning a gas stream by enlarging and collecting dustparticles through vapor nucleation and inertial condensation on acooler.

A further reference relating to the removal of dust from a gas stream isFR 2600553. Herein an improved gas-washing (scrubbing) process isdescribed. In a first washing step, a washing fluid is sprayed into thegas stream, in a direction countercurrent to that of the gas stream(which is the conventional direction in a washing operation). The gasstream is thereafter passed through a plurality of parallel venturinozzles, subjected to liquid/gas separation, and passed through asprayed washing fluid.

A reference directed to cleaning gas mixtures which might contain dustfrom a urea plant is EP 0 084 669. Disclosed is applying an aqueouswashing solution to which formaldehyde is added before it is brought incontact with the gas mixture. The method as disclosed specificallypertains to the addition of formaldehyde, and is carried out usingstandard scrubbers.

U.S. Pat. No. 3,985,523 concerns the removal of contaminants from airgenerated in the production of fertilizers. Disclosed is a processwherein contaminated vapor is condensed and a resulting liquidcontaminant stream is further treated.

Of particular concern to those in the field of air pollution control isthe reduction of emitted “fine particulate” due to the adverse healtheffects associated with both long-term and short-term respiratoryexposure to fine particulate. As used herein, the term “fineparticulate” should be understood to mean particles having a diametersmaller than 2.5 82 m. In an effort to control these particles, the EPAhas recently reduced the “PM2.5 standards” for the emissions ofparticles less than 2.5 μm. These small particles are difficult tocollect in conventional scrubbers due to their size. Nonetheless,particles in this size range are currently responsible for the measuredemissions.

Urea dust is soluble in water. When solid particles of urea are capturedin water, they fully or partially dissolve into a solution of water andurea. As increasingly more urea is captured in water, the concentrationof dissolved urea will increase until a solubility limit is reached andno further urea will be dissolved. As thermodynamic conditions change,urea can also precipitate out of solution, forming solid particles. Whencapturing urea dust in a scrubber, it is beneficial to concentrate andcontrol the urea concentration of the solution so that the captured ureacan be beneficially reused.

The prior art does not relate to processes by which condensationscrubbing of off-gas from urea finishing can be suitably conducted.Particularly, the prior art does not teach how to overcome the problemthat in urea finishing, such as in urea-granulation technology, it hasbeen recognized to be impossible, in practice, to obtain asupersaturated gas-stream downstream of the finishing step, let alone toreach supersaturation.

It is now desired to provide a method for treating the off-gas of a ureafinishing section in such a way as to effectively remove urea dust. Itis further desired to provide a method by which this removal isimproved. And, moreover, it is desired to achieve this in a process ofimproved energy efficiency.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention, in one aspect, presents a method for removing particles (alsoindicated as particulate matter or dust) from a gas stream obtained froma finishing section of a urea production plant, the method comprisingquenching said gas stream using a first aqueous stream so as to create aquenched gas stream; mixing said quenched gas stream with ahumidification fluid selected from (a) saturated steam and (b)superheated steam mixed with a second aqueous stream, so as to produce ahumidified gas stream, subjecting said humidified gas stream to particleseparation (i.e., dust removal) by means of a scrubbing liquid in whichat least part of the particles in the gas stream are soluble.

In another aspect, the invention provides a finishing equipment for aurea plant, said finishing equipment comprising a urea finishing devicecomprising an inlet for liquid urea, an inlet for cooling gas, acollector for solid urea, an outlet for off-gas, said outlet being influid communication with a gas treatment section comprising, indownstream order, a quenching zone provided with an inlet for aquenching liquid, a humidification zone comprising an inlet for steam orfor steam mixed with an aqueous stream, and a particle removal system(also indicated as a dust removal system) comprising an inlet for ascrubbing liquid and an outlet for an aqueous stream with dissolvedparticles.

In a further aspect, the invention concerns a urea plant comprising asynthesis and recovery section; said section being in fluidcommunication with an evaporation section, said evaporation sectionbeing in fluid communication with a finishing section and having a gasflow line to a condensation section; wherein said finishing section hasa gas flow line to a gas treatment section comprising, in downstreamorder, a quenching system provided with an inlet for a quenching liquid,a humidification system comprising an inlet for steam or for steam mixedwith an aqueous stream, and a particle removal system comprising aninlet for a scrubbing liquid and an outlet for an aqueous stream withdissolved particles.

In yet another aspect, the invention presents a method of modifying anexisting urea plant, said plant comprising a synthesis and recoverysection (A); said section being in fluid communication with anevaporation section (B), said evaporation section being in fluidcommunication with a finishing section (C) and having a gas flow line toa condensation section (E); said finishing section (C) having a gas flowline to a dust scrubbing section (D), wherein one modifies the plant byplacing, between the finishing section (C) and the dust scrubbingsection (D), a gas treatment section comprising, in downstream order, aquenching system (F) provided with an inlet for a quenching liquid, anda humidification system (G) comprising an inlet for steam or for steammixed with an aqueous stream, and a gas outlet that is in fluidcommunication with said dust scrubbing section (D).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of a urea plant provided with a gastreatment section according to the invention;

FIGS. 2-6 depict flow schemes of embodiments of a process according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

In a broad sense, the invention is based on the judicious insight topurify the off-gas of a urea finishing section by subjecting it to adouble treatment, viz. quenching and humidification. Particularly, theinvention is based on the judicious insight to thereby apply ahumidification fluid selected from (a) saturated steam and (b)superheated steam mixed with an aqueous stream. An advantage of thetreatment of the invention is that the humidification can be conductedso as to bring about saturation of the off-gas, despite the dry naturethereof. This has hitherto not been conceivable in the art for a dry gasstream such as the off-gas from a finishing section of a urea plant.More particularly, the gas stream to be treated is an exhaust gas streamfrom a fluid bed granulator or a prilling tower in a urea productionprocess. However, the inventors believe that the method is alsoapplicable to other gas streams having the same dry character. Thus, theinvention can also be applied to other gas streams that aresubstantially water free, such as having less than 1% of water.

The first step of the aforementioned double treatment is quenching. Itis noted that, as the skilled person will understand, quenching of a gasstream is fundamentally different from washing. The purpose of quenchingis to condition the gas stream, particularly by generating an atmospherehaving a 100% relative humidity (RH). Typically, this is done byspraying a quenching liquid co-currently with the gas stream, and/or toprovide a quenching chamber wherein the gas and the quenching liquid aresubjected to a residence time that is sufficiently long for the gas tobe conditioned at, or at least close to 100% RH. A washing (orscrubbing) operation, on the other hand, is not related to conditioningan atmosphere, but to bring about a physical contact between a gas to bewashed (i.e. scrubbed) and a washing liquid, after which an immediateremoval of the washing liquid is normally foreseen. Typically,therefore, the washing (scrubbing) of a gas stream involves contactingthe gas with a counter-current or cross-current flow of a washingliquid.

It will also be understood that liquids used for washing a gas stream,due to the different purpose of the liquid, are not applied in such away as to induce quenching of the gas.

Quenching refers to adding water to the off-gas. This is generally doneby one or more quenchers, i.e. devices that serve to introduce waterinto the gas stream. This introduction will generally be done in such away that the water is well-dispersed into the gas. Preferably, the wateris introduced into the gas by spraying it into the gas flow line betweenthe finishing section and the dust scrubbing section. This can be doneby spraying liquid into a duct just preceding the dust scrubbingsection. It can also be a separate chamber or tower equipped with aspray system. Spray systems, suitable atomization nozzles, and the like,are known to the skilled person. Preferably, the liquid is sprayed insuch a way and consistency that liquid droplets are formed that are sosmall that the droplets evaporate quickly and a liquid saturation in thevapor near equilibrium is reached within a short time. The quenching canbe done as a single operating step, or in two or more stages. In suchstages, the quenching liquids can be the same or different.

The quenching, as applied in the present invention, of the off-gas froma finishing section of a urea plant, also results in cooling down thegas. This is preferably to a temperature below about 45° C. and creatinga liquid saturation near equilibrium. Preferably, the liquid is sprayedin such a way and consistency that liquid droplets are formed that areso small that the droplets evaporate quickly, and a liquid saturation inthe vapor near equilibrium is reached within a short time. Forcompleteness' sake, it is noted that spraying the aqueous quenchingliquid only, does not lead to complete saturation, let alonesupersaturation.

Preferably the quenching stream (i.e., the first aqueous stream) has atemperature of below 45° C., more preferably below 40° C., mostpreferably below 35° C. The typical air temperature of the off-gasexiting a finishing section of a urea plant, such as in fluid bedgranulation, is about 100° C. After quenching, the temperature ispreferably below 45° C. Accordingly, the temperature of the gas streamis lowered by typically more than 50° C., preferably more than 60° C.,and most preferably more than 65° C. It will be understood that in theevent of gas streams other than off-gas from urea finishing, theabsolute temperature values can be different. Further, the skilledperson will understand that the cooling depends on the amount of liquidwhich can be evaporated. This is determined by the temperature and watervapor content of the off-gas to be quenched.

The quenching liquid will generally be a re-used process liquid, eitherfrom a nearby installation or a plant coupled to the plant in which thegas stream is to be treated, or from a different part of the same plant.More preferably, the quenching liquid is re-circulated from the gastreatment itself, as explained below.

The second step of said double treatment is humidification, by applyingeither saturated steam or superheated steam mixed with an aqueousstream. The humidification of the quenched gas stream allows bringingabout saturation of the gas stream. Preferably, therefore thehumidification is, in fact, saturation.

A next step comprises subjecting the humidified, preferably saturatedgas stream to particle separation. This is done, in one or more steps,by means of a scrubbing liquid in which at least part of the particlesin the gas stream are soluble. Typically, the scrubbing liquid is wateror an acidified solution, such as a solution acidified with for examplesulphuric acid.

All or at least part of the captured particulate matter dissolves in thescrubbing liquid. Typically, 0.1 wt. % to 99.9 wt. % of the capturedsoluble particulate dissolves in the quenching liquid. Preferably, atleast 50 wt. % of the captured particulate dissolves in the scrubbingliquid, e.g. 50 wt. % to 95 wt. %, preferably 80 wt. % to 95 wt. %.

In an interesting embodiment, the first aqueous stream, i.e., the streamused as a quenching liquid, comprises dissolved particles which are thesame as the particles to be removed from the gas stream. Preferably,thereby the quenching liquid comprises an aqueous liquid recycled fromthe scrubbing step. This refers to an aqueous stream obtained fromscrubbing the humidified gas stream. This stream will comprise dissolvedparticles as a result of the removal of dust from the scrubbed gasstream. Without wishing to be bound by theory, the inventors believethat the presence of dissolved particles in the quenching liquid willaid the growing of such particles when further dust particles areretrieved from the gas stream. This, in turn, facilitates the removal ofsuch dust particles from the gas stream.

A further interesting embodiment relates to the aforementioned option(b), whereby the humidification fluid comprises superheated steam mixedwith a second aqueous stream. In this embodiment it is preferred thatsaid second aqueous stream has the same composition as the first aqueousstream.

In a further preference hereof, both the first and the second aqueousstream comprise dissolved particles which are the same as the particlesto be removed from the gas stream. In that event, both of these streamspreferably comprise an aqueous liquid recycled from the scrubbingliquid, i.e., the liquid obtained after the step of scrubbing of thehumidified gas stream. Thereby the second aqueous stream, i.e. thestream used in the humidification step, preferably has a lowerconcentration of dissolved particles than the first aqueous stream.

In the event of the treatment of a gas stream obtained as off-gas from aurea finishing section, typically ammonia is present in the gas streamto be treated. Further, typically dissolved ammonium salts, traces ofacid, or both, are present in the aqueous stream obtained afterscrubbing, particularly as generated by acidic scrubbing of thehumidified gas stream.

In an interesting embodiment, the particle separation comprises one ormore venturi scrubbing steps. A venturi scrubber is a well known type ofdevice for removing contaminants from a gaseous effluent stream. venturiscrubbers are generally recognized as having the highest fine particlecollection efficiency of available scrubbing devices. In a venturiscrubber the effluent gas is forced or drawn through a venturi tubehaving a narrow “throat” portion. As the air moves through the throat itis accelerated to a high velocity. A scrubbing liquid in the form ofdroplets, typically of water, is added to the venturi, usually at thethroat, and enters the gas flow. The water droplets used are generallymany orders of magnitude larger than the contaminant particles to becollected and, as a consequence, accelerate at a different rate throughthe venturi. The differential acceleration causes interactions betweenthe water droplets and the contaminant particles, such that thecontaminant particles are collected by the water droplets. Thecollection mechanisms involve, primarily, collisions between theparticles and the droplets and diffusion of particles to the surface ofthe droplets. In either case, the particles are captured by thedroplets. Depending on the size of the contaminant particles, one or theother of these mechanisms may predominate, with diffusion being thepredominant collection mechanism for very small particles, and collisionor interception being the predominant mechanism for larger particles. Aventuri scrubber can also be efficient at collecting highly solublegaseous compounds by diffusion. A detailed description of thesescrubbing mechanisms is discussed in Chapter 9 of Air Pollution ControlTheory, M. Crawford, (McGraw-Hill 1976).

Where it is spoken of “venturi scrubber” this can refer to either asingle venturi scrubber or a plurality of venturi scrubbers. Further,one or more venturi scrubbers can themselves comprise one or moreventuri tubes.

Where, in this description, it is spoken of “fluid communication”, thisrefers to any connection between a first part or section of a plant anda second part or section of a plant via which fluids, notably liquids,can flow from the first part of the plant to the second part of theplant. Such fluid communication is typically provided by piping systems,ducts, hoses, or other devices well-known to the skilled person for thetransportation of fluids.

Where in this description it is spoken of “gas flow lines” this refersto any connection between a first part or section of a plant and asecond part or section of a plant via which gas or vapors, notablyaqueous vapors, can flow from the first part of the plant to the secondpart of the plant. Such gas flow lines typically comprise pipingsystems, ducts or other devices well-known to the skilled person for thetransportation of gases, if needed under above or below (vacuum)atmospheric pressures.

When in this description it is spoken of “zone” or “section” this refersto generally the same thing, viz. a part of an industrial installation,where certain defined events occur. Such a part may also be indicatedwith the term “system.”

The invention particularly pertains to urea finishing. This part of aurea production process refers to the section where solid urea isobtained.

As an example, a schematic drawing of a plant having a finishing sectionin accordance with the invention is depicted in FIG. 1. For convenience,parts of the plant discussed below refer to the elements contained inFIG. 1. This does not imply that any plant built in accordance with theinvention needs to be in accordance with FIG. 1.

This finishing section, section (C) in FIG. 1, may be a prilling tower,granulation section, pelletizing section, or a section or equipmentbased on any other finishing technique. A granulation section may be afluidized bed-granulation, or a drum granulation, or a pan-granulation,or any other similar and known granulation device. The main function ofthis finishing section is to transfer a urea melt, as obtained from ureasynthesis, into a stream of solidified particles. These solidifiedparticles, usually prills, granules or pellets, is the main productstream from the urea plant. In any event, to transfer the urea from theliquid phase into the solid phase, the liquid has to be cooled down tothe crystallization temperature and the heat of crystallization has tobe removed. Moreover, usually some additional heat is removed from thesolidified urea particles, in order to cool them to a temperature thatis suitable for further processing and handling, including safe andcomfortable storage and transport of this final product. The resultingremoval of the total amount of heat for cooling the liquid tocrystallization temperature, for transfer from liquid into solid phaseand for subcooling of the solid particles in the finishing section, isusually done by air picking up heat and leaving the finishing section ata temperature higher than the inlet temperature. Liquid sprayed into theair evaporates before the air enters the zone in which solidificationtakes plat. It cools down the air, and this way it decreases the airrequirement.

Usually most of the crystallization/cooling heat is removed by coolingwith air. The cooling air, by nature of the cooling process, leaves thefinishing section at an increased temperature. Usually an amount of airequal to 3-30 kg of air per kg of final solidified product is applied,preferably 3-10 kg. This is the typical off-gas of the finishing sectionof a urea production plant.

In the finishing section (C), the air comes into direct contact with theurea melt and with the solidified urea particles. This inadvertentlyleads to some contamination of the air with some urea dust, and ammonia.Depending on the nature of the finishing section (prilling/granulation,type of granulation, conditions selected in granulation), the amount ofdust present in the air may vary widely, values being in the range of0.05% to 10% by weight(with respect to the final product flow) havingbeen observed. For a finishing section based on granulation, the amountof dust more typically is in a range of from 2% to 8% by weight. Thispresence of dust in the off-gas usually makes a dust removal system (D)required, either for environmental or from economical considerations,before the air can be vented back into the atmosphere.

In the dust scrubbing section (D), dust scrubbing is usually done usinga circulating urea solution as a washing agent. On top of this alsofresh water scrubbing usually is applied. In the dust scrubbing sectionD a purge flow of urea solution is obtained. This purge flow usually hasa concentration of 10-60% (by wt.) of urea. In order to reprocess theurea present in this purge flow, the purge flow is returned to theevaporation section (B), where it is further concentrated and thenrecycled to the finishing section (C). Cleaned air is vented from thedust scrubbing into the atmosphere. In the dust scrubbing section, e.g.,one or a combination of the following wet scrubbing technologies can beapplied: spray chamber scrubbing, packed bed scrubbing, impingementplate scrubbing, mechanically-aided scrubbing, venturi scrubbing,orifice scrubbing, condensation scrubbing, charged scrubbing, fiber-bedscrubbing.

A quench zone employing spray quenchers will preferably comprise (a) asection in which the gas to be quenched is cooled by the introduction(e.g. injection) and evaporation of water; (b) a particulate matter(dust) capture basin, serving to collect dust stripped from the gas; (c)a sprayer system consisting of lances equipped with injection nozzles,and (d) a water supply system with pumps.

Before makeup water is added to the aqueous quenching liquid, thesolution concentration is generally allowed to cycle up by recirculationof the quenching liquid. The latter is also a standard choice for theskilled person seen from process economy. Generally, quenching liquid isre-circulated until the dissolved particulate solution reaches aconcentration of up to 50% by weight before it is extracted or bled off.In practice, a portion of the circulating fluid is continuouslyextracted containing the desired concentration of the dissolvedparticulate matter. This extracted liquid is sometimes called the purgeor blow-down. At the same time, the remaining liquid is diluted byaddition of makeup water which can be fresh water or a more dilutedstream (e.g., from a section downstream of the quench).

The off-gas (or “gaseous effluent”) coming from the finishing section,e.g. from a prilling tower or fluid bed granulator, is intended toinclude effluent streams that have liquid or solid particulate materialentrained therein, including vapors which may condense as the effluentstream is cooled.

In the quench zone, the gaseous effluent is cooled to a much lowertemperature, preferably below about 45° C. Many methods of cooling a hoteffluent gas flow are known to those skilled in the art.

A preferred method for use in the invention involves spraying a coolingliquid such as water, into the gas through nozzles. Without wishing tobe bound by theory, the inventors believe that spray-quenchingcontributes to the efficient removal of dust, by allowing water tointeract with dust particles.

This is an unexpected benefit of spray-quenching. In the art, notrelated to urea but, e.g., to flue gas, cooling of a gaseous effluenthas an effect in supersaturated systems. Therein, cooling the effluentcauses condensable vapors in the effluent stream to undergo phasetransition. Condensation of these vapors will naturally occur aroundparticles in the effluent stream which serve as nucleation points.Pre-cooling the effluent stream is, thus, useful for two reasons. Firstcondensable contaminants are transformed to the liquid phase and arethereby more easily removed from the effluent. Second, the nucleationprocess increases the size of pre-existing particles in the effluent,thereby making it easier to remove them.

The removal of the larger particles by quenching prevents the largerparticles from competing with the submicron particles as nucleationsites. As mentioned above, it is desirable that the submicron particlesincrease in size due to condensation so that they are easier to removefrom the effluent flow.

A problem with the gaseous effluent treated in the invention, i.e. theoff-gas of a finishing section of a urea plant, is that it is in asubsaturated state. As the sole condensable vapor, the off-gas containsa limited amount of water. As a result, it would have to be cooled downto much lower values than achievable by quenching in order to have watercondense as desired. It is noted that it is fundamentally impossible tocool down the off-gas, by quenching only, to such a low temperature thatcondensation of water vapor will occur, because heat is only removed byevaporation of water. Starting from a subsaturated state, quenching canlower the temperature and increase the water vapour content of theoff-gas only until the point of full saturation. This point ofequilibrium between off-gas and quenching liquid cannot be crossed, orin other words supersaturation cannot be achieved. The invention solvesthis by introducing the additional step of humidification.

The quenched and humidified gas is led to a particle capture zone. A“particle capture zone” refers to a section in which the gas issubjected to conditions serving the removal of particulate mattertherefrom. Typically, this refers to a particle capture vessel such as awet scrubber. It can also refer to, e.g., a venturi scrubber or a wetelectrostatic precipitator (WESP). In a preferred embodiment, theparticle capture zone comprises a combination of, in series, a wetscrubber (such as a tray scrubber) and, downstream thereof, a venturiscrubber. More preferably, the venturi scrubber comprises a plurality ofventuri tubes in parallel. In another preferred embodiment, a WESP ispositioned downstream of the wet scrubber, or downstream of the venturiscrubber, or most preferably in series after the wet scrubber and theventuri scrubber.

The invention also pertains to the equipment for carrying out theabove-described method. This refers to a finishing system for a ureaplant. Therein a urea finishing device is present comprising theappropriate attributes to perform its function. These attributes areknown to the skilled person, and generally include an inlet for liquidurea, an inlet for cooling gas, a collector for solid urea (typically:urea particles, preferably granules), and an outlet for off-gas. Theoutlet for off-gas is in fluid communication (typically via a gas flowline) with the inlet of a gas treatment section comprising, indownstream order, a quenching system provided with an inlet for aquenching liquid, a humidification system comprising an inlet for steam,or a mixture of steam and an aqueous stream, and a particle removalsystem comprising an inlet for a scrubbing liquid and an outlet for anaqueous stream with dissolved particles.

In one embodiment, the steps of quenching and humidification are carriedout in sequence, but in the same equipment. In another embodiment, thesesteps are carried out in separate equipment.

In a preferred embodiment, the particle removal system comprises aplurality of venturi scrubbers, operated in parallel. Preferably, thedust removal system is so designed that these parallel venturi tubes canbe operated independently of each other, i.e. the number of venturitubes used at the same time, can be adapted during the process asdesired. A preferred system is that provided by Envirocare.

Envirocare scrubbers consist of a quenching section, downstream of whicha so-called MMV-section (micro-mist venturi) is installed. TheMMV-section consists of multiple parallel venturis. In the MMV-sectionlarge quantities of liquid are sprayed in the throat of the venturisco-currently with the gas-flow through single phase nozzles, creating aconsistent and adjustable liquid droplet-size, typically in a range offrom 50 μm to 700 μm. The liquid droplet size is one of the parametersthat can be used to control the efficiency of dust-removal

The invention also pertains to a urea plant comprising a finishingsection as described above. More particularly, the urea plant of theinvention, as illustrated in the example of FIG. 1, comprises asynthesis and recovery section (A); which is in fluid communication withan evaporation section (B). The evaporation section is in fluidcommunication with a finishing section (C), and has a gas flow line to acondensation section (E). The finishing section (C) has a gas flow lineto a gas treatment section comprising, in downstream order, a quenchingsystem provided with an inlet for a quenching liquid (F), ahumidification system (G) comprising an inlet for steam, or a mixture ofsteam and an aqueous stream, and a particle removal system (D)comprising an inlet for a scrubbing liquid and an outlet for an aqueousstream with dissolved particles.

In a preferred embodiment, the dust scrubbing section comprises at leastone venturi scrubber (D), and the quenching system preferably comprisesa spray-quencher (F). The quenching system is installed between thefinishing section (C) and the venturi scrubber (D), and is in fluidcommunication with the gas flow line between the finishing section (C)and the humidification section (G). Preferably, a plurality of venturiscrubbers is employed as outlined above. It will be understood that anydesired number of venturis is in fluid communication (typically via agas flow line) with the gas outlet of the finishing section.

The invention is applicable to the construction of new urea plants(“grass root” plants) as well as in revamping existing urea plants.

It will be understood that a new plant according to the invention canjust be built in conformity with the above. In revamping existingplants, the invention pertains to a method of modifying an existing ureaplant, in such a way as to ensure that the plant has a gas treatmentsection provided with, in downstream order, a quenching system providedwith an inlet for a quenching liquid, a humidification system comprisingan inlet for steam, or a mixture of steam and an aqueous stream , and aparticle removal system comprising an inlet for a scrubbing liquid andan outlet for an aqueous stream with dissolved particles.

Hereinafter several embodiments of the invention will be discussed withreference to the drawings. It will be understood that the invention isnot limited to the embodiments shown in any of the drawings.Accordingly, the following descriptions of embodiments, whilstdiscussing the components shown in FIGS. 1 to 5 are also applicable toother embodiments, not necessarily as shown.

As an example, a schematic drawing of a plant having a finishing sectionin accordance with the invention is depicted in FIG. 1. The blockdiagram in FIG. 1 shows a urea plant comprising (A) a synthesis andrecovery section. Said section is in fluid communication with anevaporation section (B). The evaporation section is in fluidcommunication with a finishing section (C), and has s gas flow line to acondensation section (E). The finishing section (C), which can comprise,e.g. a prilling tower, a granulation section, a pelletizing section, ora section or equipment based on any other finishing technique. Agranulation section may be a fluidized bed-granulation, or a drumgranulation, or a pan-granulation, or any other similar and knowngranulation device. Conventionally, the finishing section (C) would havea gas flow line to a dust scrubbing section (D). The plant according toFIG. 1 has been modified in accordance with the invention. As a result,between the finishing section (C) and the dust scrubbing section (D), agas treatment section is placed. Said gas treatment section comprises,in downstream order, a quenching section (F) provided with an inlet fora quenching liquid, and a gas outlet to a humidification section (G).Thehumidification section comprises, in addition to a gas inlet connectedto the gas outlet of the quenching section, an inlet for steam, or for amixture of steam and an aqueous stream, and a gas outlet that is influid communication with said dust scrubbing section (D).

FIGS. 2-6 provide further detailed schemes of process embodiments of theinvention. The legend to FIGS. 2-5 is as follows:

Sections:

-   H. Quenching zone;-   I. Humidification zone;-   J. Particle removal zone;-   K. Evaporation zone;

Streams:

-   (a) gas with dust particles, 100° C., low humidity;-   (b) quenched gas 45° C., 80% RH;-   (c) humidified gas;-   (d) saturated steam;-   (e) process water;-   (f) water with dissolved particles;-   (g) gas to atmosphere;-   (h) heat;-   (i) aqueous stream from quench-   (j) vapor;-   (k) concentrated solution;-   (l) superheated steam;-   (m) aqueous stream;-   (n) water with reduced concentration of dissolved particles;-   (o) water with increased concentration dissolved particles;

It is shown that gas with dust particles, of a temperature of 100° C.and of low humidity (typically a dry off-gas, of relatively lowtemperature, of a finishing section of a urea production plant) is fedto a gas treatment section. This involves first subjecting the gas to aquench, whereby the gas becomes cooled down to 45° C. and whereby therelative humidity (RH) of the quenched gas is increased to 80% RH. Thisquenched gas is then subjected to humidification, thereby bringing aboutsaturation. In the embodiment of FIG. 2, this is by being brought intocontact with saturated steam.

As the skilled person will understand, the amount of steam (i.e., theratio of steam to gas) mainly depends on the degree of saturation whichcan be achieved in the preceding quench. A typical range is 0.5-5%relative to the amount of gas (air), e.g. by weight. By way of guidancein the form of an example: in the event that the water concentration inthe blowdown/purge stream is fixed at, e.g., 55%, the RH of the gasleaving the quench is 80.6%. Then 3116 kg/h of steam is required, whichis 3% relative to the amount of gas (air). At 80% water concentration,the RH of the gas is 93.5% and only 736 kg/h of stem is required toachieve 100% saturation. 736 kg/h is 0.7% relative to the amount of gas.

In the embodiment of FIG. 3, this is by being brought into contact withsuperheated steam and with an aqueous stream (i.e., a second aqueousstream). The superheated steam will generally have a temperature in arange of from 125° C. to 250° C., more typically from 140° C. to 200° C.The amount of said second aqueous stream will generally vary between 5%and 15% relative to the superheated steam, more typically from 7% to12%.

The resulting humidified (and saturated) gas stream is then subjected tothe removal of dust particles, by scrubbing with process water, i..e,with a stream recycled from elsewhere in the plant (in the event of aurea plant, the water supplied to the scrubbing system typically is acondensate from the wastewater treatment).

The resulting scrubbed gas is sent into the atmosphere. The obtainedscrubbing liquid, in this embodiment, is recycled to the quenchingsection, and used as a quenching liquid.

In FIG. 4, the above-mentioned second aqueous stream applied in thehumidification section, is the same recycled stream of the scrubbingliquid as used in the quenching section.

FIG. 5 is similar to FIG. 4, but herein the scrubbing liquid is recycledin two different ways. From the scrubbing section, scrubbing liquidswith different concentrations of dissolved particles are extracted. Thescrubbing liquid having the higher concentration of dissolved particlesis recycled to the quenching section, as a quenching liquid (i.e., thefirst aqueous stream as applied in the process of the invention). Theextracted scrubbing liquid having the lower concentration of dissolvedparticles is recycled to the humidification section as an aqueous streamto be fed to that section when this section also receives superheatedsteam (i.e., the second aqueous stream as applied in the process of theinvention in the alternative b) for the humidification liquid).

FIG. 6 refers to a preferred embodiment wherein a lean recycle solution(i.e., having a relatively low concentration of dissolved particles) isused. Therein, following a quench, humidification and coarse particulateremoval processes, a lean recycle solution is combined with superheatedsteam in a two-phase nozzle(s) and mixed with the off gas to achieve astate of (super) saturation. Coarse particles are those having a sizegenerally above 10 μm, which are generally considered to benon-respirable, and therewith less dangerous for people's health thanparticles having a size smaller than 10 μm, which can penetrate into thelungs.

This leads to condensation of water on the fine particulate matter inthe gas stream. The large condensed droplets can then be removed fromthe off gas stream during the fine particulate and/or ammonia removalstage.

Atomization (using high pressure, ultrasound, compressed air, etc.) ofthe lean recycle solution into fine droplets is advantageous from thepoint-of-view that the atomized droplets are more easily evaporated bythe added steam. In this manner, the sensible heat of the steam does notheat the off-gas (which is less preferred as this does not facilitateachieving saturation) but for evaporation of the atomized droplets. Thisevaporative cooling not only avoids temperature increase of the off gas,but it also generates additional water vapor, thus reducing the steamconsumption even further.

The legend for FIG. 6 is as follows:

Functions:

I. Fluidised Bed Granulation;

II. Off Gas Quench & Humidification;

III. Coarse Particulate Removal;

IV. Dissolving;

V. Ammonium Salt Solution Removal;

VI. Off Gas (Super) Saturation;

VII. Fine Particulate Growth;

VIII. Fine Particulate Removal;

IX. Ammonia Removal

X. (Dedicated) Recycle Evaporation;

Streams:

1. urea melt;

2. fluidisation Gas;

3. Granulation Off Gas (Particulate +Ammonia);

4. Quenched gas;

5. Coarse particulate;

6. Quenched gas with coarse particulate removed;

7. Saturated) Steam;

8. Demineralised/Deionised Water;

9. Lean Solution;

10. Process Condensate;

11. Concentrated Solution;

12. Acidic Scrubbing Solution

13. Ammonium Salt Solution;

14. Evaporation Condensate;

15. Saturated Stack Gas;

16. Stack Condensate;

17. Emission

The lean recycle solution can be derived from such sources as:

-   -   (a) steam or process condensate used for wet scrubbing of fine        particulate,    -   (b) evaporation condensate from a dedicated evaporation process        used to concentrate and recycle recovered particulate, and    -   (c) off gas stack condensate following a particulate removal        (scrubbing) process

For enhanced scrubbing performance, the lean recycle solution can besubstituted by demineralized/deionized water. The main benefit of usingdemineralized/deionized water is seen to be related to condensationeffects rather than quenching. Any impurities present in the gas streamwill act as nuclei for condensation. The usage ofdemineralized/deionized water reduces the number of impurities added tothe gas stream such that condensation occurs on the dust particles andnot any additional impurities.

The invention is not limited to any particular urea production process.

A frequently used process for the preparation of urea according to astripping process is the carbon dioxide stripping process as for exampledescribed in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27,1996, pp 333-350. In this process, the synthesis section followed by oneor more recovery sections. The synthesis section comprises a reactor, astripper, a condenser and a scrubber in which the operating pressure isin between 12 and 18 MPa and preferably in between 13 and 16 MPa. In thesynthesis section the urea solution leaving the urea reactor is fed to astripper in which a large amount of non-converted ammonia and carbondioxide is separated from the aqueous urea solution. Such a stripper canbe a shell and tube heat exchanger in which the urea solution is fed tothe top part at the tube side and a carbon dioxide feed to the synthesisis added to the bottom part of the stripper. At the shell side, steam isadded to heat the solution. The urea solution leaves the heat exchangerat the bottom part, while the vapor phase leaves the stripper at the toppart. The vapor leaving said stripper contains ammonia, carbon dioxideand a small amount of water. Said vapor is condensed in a falling filmtype heat exchanger or a submerged type of condenser that can be ahorizontal type or a vertical type. A horizontal type submerged heatexchanger is described in Ullmann's Encyclopedia of IndustrialChemistry, Vol. A27, 1996, pp 333-350. The heat released by theexothermic carbamate condensation reaction in said condenser is usuallyused to produce steam that is used in a downstream urea processingsection for heating and concentrating the urea solution. Since a certainliquid residence time is created in a submerged type condenser, a partof the urea reaction takes already place in said condenser. The formedsolution, containing condensed ammonia, carbon dioxide, water and ureatogether with the non-condensed ammonia, carbon dioxide and inert vaporis sent to the reactor. In the reactor the above mentioned reaction fromcarbamate to urea approaches the equilibrium. The ammonia to carbondioxide molar ratio in the urea solution leaving the reactor isgenerally in between 2.5 and 4 mol/mol. It is also possible that thecondenser and the reactor are combined in one piece of equipment. Anexample of this piece of equipment as described in Ullmann'sEncyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. Theformed urea solution leaving the urea reactor is supplied to thestripper and the inert vapor containing non-condensed ammonia and carbondioxide is sent to a scrubbing section operating at a similar pressureas the reactor. In that scrubbing section the ammonia and carbon dioxideis scrubbed from the inert vapor. The formed carbamate solution from thedownstream recovery system is used as absorbent in that scrubbingsection. The urea solution leaving the stripper in this synthesissection requires a urea concentration of at least 45% by weight andpreferably at least 50% by weight to be treated in one single recoverysystem downstream the stripper. The recovery section comprises a heater,a liquid/gas separator and a condenser. The pressure in this recoverysection is between 200 to 600 kPa. In the heater of the recovery sectionthe bulk of ammonia and carbon dioxide is separated from the urea andwater phase by heating the urea solution. Usually steam is used asheating agent. The urea and water phase, contains a small amount ofdissolved ammonia and carbon dioxide that leaves the recovery sectionand is sent to a downstream urea processing section where the ureasolution is concentrated by evaporating the water from said solution.

Other processes and plants include those that are based on technologysuch as the HEC process developed by Urea Casale, the ACES processdeveloped by Toyo Engineering Corporation and the process developed bySnamprogetti. All of these processes, and others, may be used precedingthe urea finishing method of the invention.

Urea finishing techniques, such as prilling and granulation, are knownto the skilled person. Reference is made to, e.g., Ullmann'sEncyclopedia of Industrial Chemistry, 2010, chapter 4.5. on urea.

The invention will be further illustrated hereinafter with reference tothe Example below. The Example is not intended to limit the invention.

EXAMPLE

A model was developed using flowsheeting software to study the effect ofthe main process parameters and show the differences between varioussteam injection and recycle stream options.

The results are given in Table 1 below.

In all cases the air flow to the quench is 100,000 kg/h. The relativehumidity after the quench is either 80.6% or 93.5%, depending on thewater concentration in the blowdown (purge), which is either 55% or 80%in this example. The relative humidity of the gas, in equilibrium withthe urea/water solution, is following said water concentration. The RHof the gas in equilibrium with a 55% water/45% urea solution is 80.6%.

Case 1 represents the embodiment of FIG. 2 with saturation with steamonly after quenching with the blowdown concentration fixed at 55% water.

Case 2 represents the embodiment of FIG. 2 with saturation with steamonly after quenching with the blowdown concentration fixed at 80% water.

Comparison between cases 1 and 2 shows that overall steam consumptioncan be reduced by diluting the quenching and blowdown concentration.This has to do with the fact that after quenching, the relative humidityis already higher for case 2 than for case 1. This is by virtue ofquenching with a more dilute solution, so less steam has to be added toachieve saturation. So in case 2 the temperature increase of the gas dueto addition of steam is much lower and the reduction of steam requiredfor saturation is larger than the increase of steam required forsubsequent evaporation for concentration of the more dilute blowdownstream.

Case 3 represents the embodiment of FIG. 4 with saturation with steamand water with dissolved particles after quenching, with the blowdownconcentration fixed at 55% water.

Case 4 represents the embodiment of FIG. 3 with saturation with steamand water without dissolved particles (demineralized water) afterquenching with the blowdown concentration fixed at 55% water.

The results from cases 3 and 4 show that there is a major differencebetween saturation with steam only and saturation with steam and wateraddition. If only steam is used (cases 1 and 2), the temperature of thegas during steam addition increases. This means that the partialpressure of water increases so more steam is required to achievesaturation. Combining a second aqueous stream (with a higher waterconcentration than the initial quench stream) with steam for saturationof the gas (cases 3 and 4) decreases steam consumption because it avoidsthe temperature increase associated with steam injection alone.

When considering overall water and steam consumption of cases 3 and 4,there is minimal difference between adding water with dissolvedparticles to the saturation step, or adding demineralized water. Case 4has an advantage over case 3 for particle growth in the gas stream. Anyimpurities present in the gas stream will act as nuclei forcondensation. The usage of demineralized/deionized water reduces thenumber of impurities added to the gas stream such that condensationoccurs on the dust particles and not any additional impurities.

Case 5 represents the embodiment of FIG. 3, with saturation with steamand water without dissolved particles (demineralized water) afterquenching with the blowdown concentration fixed at 80% water.

Comparison between cases 3, 4 and 5 shows that diluting the blowdownfrom the quench increases the overall steam consumption. This iscontrary to the comparison of cases 1 and 2, where diluting the blowdownconcentration decreased the overall steam consumption.

The amount of steam required for saturation decreases for case 5 incomparison to cases 3 and 4. However, this reduction is much lower thanthe reduction of case 2 over case 1 because the temperature increase dueto steam addition is already mitigated by injecting water in thesaturation section.

The amount of steam required for evaporation/concentration is higher forcase 5 than cases 3 and 4. This increase of required steam outweighs thebenefit of reduced steam consumption for saturation.

Saturation Steam & Steam & demin. with water + Steam & water toSaturation steam, more dissolved demin. saturation, with steam makeupparticles to water to more makeup only water saturation saturation waterstream parameter unit case 1 case 2 case 3 case 4 case 5 water +dissolved urea to quench 205 H2O kg/h water 2888.1 4097.2 2866.6 2866.64089.7 steam to saturation 222 H2O kg/h steam 3115.9 735.8 1142.2 1142.8336.4 demineralized water to saturation 221 H2O kg/h water 0.0 0.0 0.094.3 25.5 water + dissolved urea to saturation 203 H2O kg/h water 0.00.0 85.1 0.0 0.0 temperature after quench 102 TEMP ° C. 43.5 41.0 43.443.4 41.0 temperature after saturation 104 TEMP ° C. 48.4 42.2 43.4 43.441.0 water in off-gas scrubbing 106 H2O kg/h water 7750.3 5479.4 5862.95863.5 5105.8 water in gas to quench 101 H2O kg/h water 2291.0 2291.02291.0 2291.0 2291.0 water uptake by gas kg/h water 5459.3 3188.4 3571.93572.5 2814.8 blowdown to evaporation section 103 H2O kg/h water 494.11617.0 493.9 494.1 1617.0 steam consumption evaporation 901 H2O kg/hsteam 664.5 2051.3 664.8 664.6 1061.4 make-up water to scrubbing 201 H2Okg/h water 2837.4 4069.6 2923.6 2839.5 4069.9 total water consumptionkg/h water 5963.4 4805.4 4065.9 4066.6 4431.8 LP steam consumption,total kg/h steam 3780.4 2787.1 1806.5 1807.4 2387.8 water balance kg/hwater 0.0 0.0 0.0 0.0 0.0

1-9. (canceled)
 10. A urea plant comprising a synthesis and recoverysection; said section being in fluid communication with an evaporationsection, said evaporation section being in fluid communication with afinishing section and having a gas flow line to a condensation section;wherein said finishing section has a gas flow line to a gas treatmentsection comprising, in downstream order, a quenching system providedwith an inlet for a quenching liquid, a humidification system comprisingan inlet for steam or for a mixture of steam and an aqueous stream, anda particle removal system comprising an inlet for a scrubbing liquid andan outlet for an aqueous stream with dissolved particles.
 11. The ureaplant according to claim 10, wherein the particle removal systemcomprises means for one or a combination of spray chamber scrubbing,packed bed scrubbing, impingement plate scrubbing, mechanically-aidedscrubbing, venturi scrubbing, orifice scrubbing, condensation scrubbing,charged scrubbing, and fiber-bed scrubbing.
 12. The urea plant accordingto claim 10, wherein the particle removal system comprises one or moreventuri scrubbers.
 13. The urea plant according to claim 12, wherein theone or more venturi scrubbers can each comprise one or more venturitubes.
 14. The urea plant according to claim 12, wherein the venturiscrubbers are multiple parallel venturis.
 15. The urea plant accordingto claim 10, wherein the particle removal system comprises one or moreof a wet scrubber, a venturi scrubber, and a wet electrostaticprecipitator.
 16. The urea plant according to claim 15, wherein hereinthe particle removal system comprises a wet scrubber and downstreamthereof a venturi scrubber.
 17. The urea plant according to claim 10,wherein the quenching system comprises a spray-quencher.
 18. The ureaplant according to claim 10, wherein the mixture of steam and an aqueousstream is a superheated steam mixed with an aqueous stream.
 19. The ureaplant according to claim 10, wherein the finishing section comprises afluid bed granulator or a prilling tower.
 20. The urea plant accordingto claim 10, wherein the quenching liquid comprises an aqueous liquidrecycled from the particle removal section.
 21. The urea plant accordingto claim 10, wherein the scrubbing liquid is water or an acidifiedsolution.